1. Field of the Invention
The present invention generally relates to micromachining techniques and, in particular, to a micromachined electromechanical device having components electrically isolated from each other via junction isolation and to a method for making the same.
2. Related Art
A microelectromechanical system (MEMS) may include a movable component, such as a spring or a flexure, for example, that moves or actuates when a voltage difference is applied between the movable component and another portion of the system. Such systems are often employed to effect sensing or actuation on a small scale. However, each MEMS is typically small, usually less than a few hundred microns in thickness, and formation of each MEMS and, in particular, the movable components within each MEMS can be difficult and expensive using conventional machining technologies.
Many specialized techniques exist for the fabrication of a MEMS. For example, in forming a MEMS, a substrate of suitable material can be etched via a suitable process, such as inductively coupled plasma reactive ion etching (ICP-RIE), for example, to form both the movable and non-movable components of the MEMS. To enable a voltage difference to be applied across the movable components, the substrate and, hence, the movable components are usually conductive or semiconductive. A semiconductive material, such as silicon, is often used to form the substrate. Indeed, single crystal silicon (SCS) is often a preferred material for use as a substrate in a MEMS, as SCS has excellent mechanical properties, such as fatigue resistance, for example, in addition to good electrical properties. Further, SCS works well with the etching techniques commonly employed in forming the movable components. There are various other advantages to utilizing SCS as a substrate material in a MEMS. These advantages should be readily apparent to one skilled in the art, and further elaboration on these advantages will not be provided herein.
Adding to the complexity of many microelectromechanical systems is the fact that each movable component formed within a substrate should be electrically isolated from other (e.g., non-movable) portions of the substrate. As previously described, a movable component of a substrate is normally formed by etching the substrate. Thus, the movable component is usually comprised of the same conductive or semiconductive material as the other portions of the substrate, and additional steps are usually required to electrically isolate the movable component from the other portions of the substrate. If the movable component is not electrically isolated, then it is not usually possible to move or actuate the movable component by applying a voltage difference across the movable component and another portion of the substrate as the voltage difference will be shorted out.
Significant research has been devoted to developing methodologies for electrically isolating a movable component within a SCS substrate of a MEMS. One methodology presently used to electrically isolate the movable component includes the step of bonding a first substrate to a handle substrate and then completely etching away all portions of the first substrate between the movable component and the non-movable portions of the first substrate. Thus, the movable component is completely separated from the non-movable portions, and the handle substrate provides mechanical support for the etched substrate and maintains alignment of the movable component with respect to the non-movable portions of the etched substrate. However, utilization of the handle substrate often introduces complexities that make the fabrication of the resulting MEMS more difficult and/or expensive. For example, in structures that require symmetry, the handle substrate may undesirably add a significant amount of mass to one side of the structure, and for fluidic structures, the handle substrate may block or impede fluid flow into and out of the structure.
A second methodology developed to electrically isolate the movable component formed within a substrate of a MEMS also includes the step of completely etching away all portions of the substrate between the movable component and the non-movable portions, as described above. Thus, the movable component is completely separated from the non-movable portions of the substrate. A material, such as silicon dioxide, for example, is then backfilled into the etched spaces of the substrate in an attempt to restore the mechanical integrity of the substrate. Therefore, the movable component and the non-movable portions are held together by the backfilled material, and a handle substrate is not necessary. However, the backfilled material often has mechanical properties that are inferior to the material of the substrate, thereby reducing the mechanical integrity of the resulting structure. Further, the mechanical integrity of the structure depends on how well the backfilled material adheres to the etched substrate. Indeed, in many structures, the bond between the backfilled material and the etched substrate is a limiting factor in the overall mechanical integrity of the structure.
A third methodology developed to electrically isolate the movable component includes the step of etching a substrate to form the movable component. However, the movable component is not completely separated from the non-movable portion of the substrate, and the non-movable portion of the substrate provides mechanical support for the movable component. After etching the substrate to form the movable component, an insulating layer is grown or deposited on the substrate. Then, conductive layers (e.g., metallic films) are deposited on the insulating layer as necessary to enable a voltage difference to be applied across the movable component and another portion of the substrate. However, the formation of the insulative and conductive layers can be a difficult and/or an expensive process. In this regard, metallization of the sidewalls or, in other words, the portions within the etched regions of the substrate is typically required to provide a suitable voltage difference for actuating the movable component. Performing photolithography or other metallization techniques within this non-planar region can be particularly problematic and difficult.
Diffusion has been used in attempts to electrically isolate, via junction isolation, portions of a micromachined device from other portions of the micromachined device. In this regard, a dopant is diffused into a layer of a microfabricated structure in order to change the electrical properties of the doped region, which resides between two regions of the layer that are to be electrically isolated from each other. More specifically, the electrical properties of the doped region are changed such that the doped region better resists the flow of electricity between the two regions that are separated by the doped region.
For example, it is well known in the art that p-type and n-type semiconductors can be formed by diffusion of appropriate dopants into semiconductive material. Further, it is well known in the art that a junction between p-type semiconductor material and n-type semiconductor material will allow electrical current to pass easily in one direction but will restrict current flow in the opposite direction. Such a junction is commonly referred to as a diode. Two properly designed diode structures formed in series, therefore, will restrict the flow of current in either direction, thereby creating a junction isolation. Unfortunately, there exists practical limitations to the use of diffusion to effect junction isolation.
In particular, a dopant usually must be diffused through the entire thickness of a layer (i.e., from a top surface of the layer to the bottom surface of the layer) in order to electrically isolate two portions of the layer. In this regard, if the dopant is diffused through only the top portion of the layer, then current is able to flow through the bottom portion of the layer. In such a case, portions of the layer residing on opposite sides of the doped region would not be electrically isolated from each other.
It is not practical to employ diffusion techniques for the purpose of junction isolation in many MEMS applications. In this regard, utilization of diffusion to effect junction isolation in silicon is normally limited to layers having a thickness of less than approximately 10 microns. This is due primarily to the relatively low diffusion coefficients (and therefore unacceptably long times required for diffusion) for electrically active, substitutional dopants in silicon. Further, the conventional diffusion process is isotropic in nature. Even if the large diffusion depths required could be achieved, the lateral spread of the dopants would be, at minimum, approximately equal to twice the depth of diffusion, resulting in unacceptably large spacing requirements between MEMS components.
Thus, as set forth hereinabove, the thickness of most substrates used to form movable components in a MEMS is normally greater than 10 microns in order to achieve the desired mechanical stability, yet utilization of diffusion to effect junction isolation in silicon is normally limited to layers having a thickness of less than approximately 10 microns. Therefore, diffusion is normally an unsuitable technique for electrically isolating a movable component of a MEMS from other portions of the MEMS. Indeed, significant research has been devoted to discovering better methodologies for electrically isolating movable components that are on the order of tens to hundreds of microns in thickness.
Thus, a heretofore unaddressed need exists in the industry for an improved methodology for electrically isolating a movable component within a MEMS, particularly a MEMS having a movable component that is tens to hundreds of microns in thickness.